The existence of residual dental adhesive subsequent to the debonding of dental appliances, such as an orthodontic bracket, has long been a problem to the orthodontist. The presence of such residual adhesive may lead to the injury of the tooth's enamel surface when abrasive or mechanical methods are used to remove the existing residual adhesive. There has, therefore, been a continuing need for an adhesive system or adhesive bonding technique which will yield a preferential bond to the metallic faying surface, yet which will still maintain a strong, environmentally durable bond between the adherends. Such a technique should yield an adhesive failure between the tooth's enamel surface and the adhesive, thus allowing the adhesive to be carried off by the dental appliance.
In the past, various approaches have been taken to alleviate the aforementioned problems. Such techniques attempt to increase the preference of adhesive bonding to the metallic faying surface through mechanical and chemical techniques. For example, chemical and photochemical etching of the metallic faying surface have been tried as a promoter of preferential bonding to metallic adherend. Other approaches have included the use of different dental appliance geometries or the use of epoxy or bis-GMA based adhesives. All of the previously mentioned techniques have met with marginal success for various reasons. For example, the technique of etching is too cumbersome with respect to processing, whereas the use of epoxy and bis-GMA based adhesive resins failed in a brittle manner, leaving adhesive behind on the tooth's enamel due the presence of the aromatic moiety of the monomer backbone.
Urethane monomers and oligomers which are terminated with readily polymerizable acrylic or methacrylic functional groups have been previously employed in order to strengthen or toughen composite dental filling materials, as shown in U.S. Pat. Nos. 4,110,184; 4,089,763; 3,862,920 and 3,825,518. The compounds which have been used were based on backbone structures which are essentially linear. The linear structure of these molecules basically matched the linear structures of the commonly used dimethacrylate resins in dental composite restoratives and orthodontic adhesives such as Bis/GMA or ethoxylated Bis/GMA or reactive diluents such as triethylene glycol dimethacrylate, as taught by U.S. Pat. Nos. 3,751,399; 3,730,947 and 3,792,531. Trifunctional monomers having non-linear structural characteristics have been employed as additives to increase cross-linking density, (U.S. Pat. No. 3,835,090) but it was observed that the incorporation of long aliphatic chains (C&gt;4) would cause undesired softening of the resulting binder resin.